Field of the Invention
The present invention relates to an electrolyte for electrochemical device, and more particularly to an electrolyte for electrochemical device comprising a boron-based additive.
Description of the Related Art
Nowadays, there is a tendency towards lighter, thinner, shorter, and smaller in electronic, information, and biomedical equipment and instrument, so it is also expected that the batteries for the same can have advantages of smaller volume, and higher storage capacity and discharge capacity able to maintained after use for long time and thereby the relevant academic circles and industries gradually pay much attention to lithium ion batteries meeting these requirements.
The major parts of lithium ion batteries comprises a positive electrode made from positive electrode material, an electrolyte, a separator, and a negative electrode, wherein said positive electrode material generally is lithium cobalt oxides, lithium nickel oxides, or lithium manganese oxides. The principle of lithium ion batteries, in sum, is charge/discharge reaction performed by insertion and extraction of Li ions between positive and negative electrodes. Said charge/discharge reaction can be concluded as the following equations, in which M is Co, Ni, or Mn, and the reaction proceeds to the right during charge and to the left during discharge.Reaction at positive electrode: LiMO2Li(1−x)MO2+xLi++xe−Reaction at negative electrode: C6+xLi++xe−LixC6 Overall reaction: LiMO2+C6Li(1−x)MO2+LixC6 
Lithium ion batteries have the advantages of lighter mass, higher density of energy, better cycle property, and higher electric power, so they are gradually applied to production requiring higher electric power, such as power tool or electric vehicle. In the consideration to the safety and cost required by batteries as sources of powering electric vehicles, lithium iron phosphate is just like cynosure for positive electrode material for lithium ion battery; however, the degradation of the performance of the lithium iron phosphate as a positive electrode material is quick under higher temperature so that the application of the lithium iron phosphate is restricted that is a problem which all companies extremely desires to solve.
U.S. Pat. No. 6,352,798B1 disclosed the effect of phenyl boron-based compound as an additive on enhancement of the conductivity of electrolyte. The conductivities of three electrolytes obtained by adding a phenyl boron-based compound to LiCF3COOLi, LiC2F5COOLi, and LiF, respectively, in DMC at a concentration ratio of 1:1 were measured. It was found that the conductivity of the electrolyte increased with the increase of the added concentration of the phenyl boron-based compound, which was not affected by the sort of lithium salts. The result apparently indicated that all of phenyl boron-based compound have the same effect on any of lithium salts. The effect was that the conductivity of the electrolyte was enhanced by enhancement of dissociation of lithium salt.
US publication patent 20060210883A1 disclosed the effect of borane-based additives added to electrolytes in different weight percentage on impedances of the lithium ion batteries. 0, 1, 3, and 5 wt % of borane-based additives were added to 1.2 M LiPF6 in EC/PC/DMC (1:1:3) and then used with anode composed of Li1+x[Ni1/3Co1/3Mn1/3]0.9O2 and natural graphite wrapped by carbon to obtain four batteries. After test, it was found that the interfacial impedance of the batteries was obviously enhanced when the added amount of said additive was achieved to 5 wt %. That was because the borane-based additive was involved to the formation of SEI films and contributed to dissolution of LiF during formation of SEI films and thereby improved the transportation of Li ions between SEI films. The result of cycle test at 55° C. also showed that the battery with less than 3 wt % of borane-based additive had better cycle life and maximal discharge power, which implied that the battery with excess added amount of borane-based additive in the electrolyte had higher interfacial impedance and discharge capacity thereof was also decreased.
Journal of The Electrochemical Society, 153 (6) (2006) A1221-A1225 disclosed that the discharge power of the battery composed of Li1+x[Ni1/3Co1/3Mn1/3]0.9O2 and natural graphite wrapped by carbon can be improved by applying Tris(pentafluorophenyl)borane (TPFPB) as an additive to electrolyte containing 1.2 M LiPF6 in EC/PC/DMC (1:1:3) since TPFPB as an additive would induce the salt in the electrolyte to completely dissociate into cations and anions to enhance the conductivity of the electrolyte. Moreover, said additive can also reduce the production of LiF in the electrolyte, wherein said LiF is not good for the mobility of Li ions and electrons. Because free Li ion in the electrolyte would be reduced by the existence of LiF in the electrolyte, which extremely affected on the conductivity of the electrolyte, so TPFPB as an additive can assist completely dissociation of salt into cation and anion ions.
Electrochemical and solid-State, 5 (11) (2002) A248-A251 disclosed TPFPB as an additive. LiPF6 can be completely dissociated into LiF+ and PF− by adding 0.1 M TPFPB to 1M LiPF6 in EC/DMC (1:1) to reduce the formation of LiF and PF5− and thereby prevent PF5− from reacting with trace amount of water in the electrolyte to produce HF. Because of avoiding the product of HF corroding LiMn2O4 as a positive electrode material, the electrochemical stability and cycle property at 55° C. of the positive electrode material can be remarkably improved.
Electrochemical and solid-State, 6 (2) (2003) A43-A46 disclosed TPFPB as an additive. 0.1 M TPFPB was added to an electrolyte containing 1 M LiPF6 in EC/DMC (1:1) and then charged and discharged to form a SEI film on the surface of the carbonaceous material (MCMB) as a negative electrode. Since TPFPB as an additive was present in the electrolyte, the stability of the SEI film can be maintained when carbonaceous material was cycled for long time. Besides, TPFPB as an additive contributed to completely dissolution of LiF, so SEI film still stably existed under higher temperature and the lithium ion batteries comprising the combination of LiN0.80Co0.15Al0.05O2 with MCMB had higher capacity and longer cycle property.
In general, the above-mentioned additives had high molecular weight and poor solubility in the organic electrolytes, and would combine with anions in the electrolyte to form bulky molecules, thereby lowering ion mobility and affecting charge/discharge property at higher rate. However, the additive of the present invention has not only better solubility, but also less effect upon ion mobility. Moreover, the additive disclosed in the present invention has a lower cost than TPFPB and better property of discharge rate and is able to effectively improve the performance of the positive electrode material under higher temperature, so it is helpful to apply lithium ion batteries to electric vehicle, thereby facilitating the development of the industries related to electric vehicle.